Blood-vessels of nerve are comparable to those of the central nervous system, and are lined by a continuous endothelium with intercellular tight junctions. They form part of the blood-nerve barrier, together with the perineurium. Vesicles within endothelial cells of the frog sciatic nerve endoneurial vasculature do not contribute to transcapillary exchange. Hypertension, induced by vascular perfusion of the nerve, damages the vascular endothelium by forming submembrane blisters, and increases permeability to intravascular tracers, horseradish peroxidase and microperoxidase. Cells of the vasculature and perineurium are capable of proliferation and forming an intact barrier after trauma to the nerve. An adequate blood supply to the nerve is required to maintain nerve function and integrity. Blood flow of the nerve was measured with a laser Doppler flowmeter and radiotracer techniques, and was shown in the rat sciatic nerve to be close to that found for white matter of the brain. Metabolism and tension were examined in single fibers of the semitendinosus muscle of the frog during fatigue produced by prolonged tetanization, and in relation to excitation-contraction uncoupling by stretch and hypertonicity. The mechanisms of uncoupling were related to energy steps in the contraction and calcium release processes.